The use of barium carbonate to remove water soluble sulfates in industrial applications is well known. Appropriate literature discussions of use and reactivity of barium carbonate as a scum control agent are given in Moody, "Effect of Particle Size and Deflocculation of Barium Carbonate on Scum Control", Am. Cerm. Soc. Bull., 44 (8) 604-607 (1965) and in Dingle, "Reactivity of Barium Carbonate of Combatting Scumming in Structural Clay Products", Am. Ceram. Soc. Bull., 46 (9) 856-863 (1967) as well as in many other articles.
One of the common uses of barium carbonate is in the manufacture of bricks or tile. During this manufacturing process, the soluble sulfates present in shale or clay migrate to the surface during the fire hardening and calcination of the composite mass. This process is called scumming. The addition of even small amounts of barium carbonate to the mix prior to fire hardening and calcination will prevent the scumming. The barium carbonate reacts with the soluble sulfates to form very insoluble carbonates and sulfate salts which do not produce scum.
There are, however, many problems with the use of barium carbonate in this application. First, due to the insolubility of barium sulfate and barium carbonate, only the exterior surfaces of the barium carbonate particles are reactive to soluble sulfate ion. This is, therefore, a limiting factor in the chemical utilization of the barium carbonate as a scum preventive reagent.
It is also well known that the reactivity of barium carbonate for this application is dependent upon its crystalline form and particle size, the nodular amorphous particles of the smallest particle sizes being the most reactive. This leads to the second problem with use of barium carbonate in this application and that is that dispersion and handling of such small particle size pigments becomes disproportionately more difficult as the particle size decreases. This arises from the fact that the surface and edge potentials increase exponentially as the particle size decreases. Therefore, decreasing the particle size causes an increase in agglomeration of individual particles and poorer dispersion characteristics. Because of this problem, commercial grades of barium carbonate often do not flow well and they have a tendency to form lumps which are difficult to disperse. The manufacturer must, therefore, optimize particle size, surface area and dispersibility. This has resulted in the products presently on the market having a utilization between 30% and 50% of the contained barium carbonate.
Several ways to solve these problems have been proposed. The addition of additives such as magnesium silicate help make the barium carbonate more free-flowing. Even this improvement, however, tends to decrease with time. Barium carbonate has been produced in a more free-flowing crystal form but these crystals either tend to be so large that there is little surface area per unit weight and, therefore, low reactivity or so small that although they are reactive they are of very low bulk density and inconvenient for use in current industrial feeders.
U.S. Pat. No. 3,322,683 teaches the use of a dispersion of barium carbonate of limited particle size and reactivity by incorporating a small amount of a hydrophilic dispersant and water. None of the prior art, however, suggests the use of an improved barium carbonate produced by precipitating barium carbonate on an inert core to improve both reactivity and dispersibility.
Generally, the barium carbonate ranges from about 10% to 90% by weight of the total product and preferably from 20% to 60% by weight of the total product.